The invention relates to an eccentric element for ground vibrators, said element including a radially movable eccentric mass arranged on a rotatable shaft which can be caused to assume a position displaced in relation to the centreline of the shaft as a result of the rotation of said shaft.
It is often desirable to use the same ground vibrator, e.g. a vibrating roller, for compacting different types of paving material. However, since compacting of stone paving, for example, will be more effective if compaction is performed with an applied oscillation of higher amplitude and lower frequency than for asphalt paving, ground vibrators intended for such a wide range of uses should be provided with the possibility of changing the amplitude and frequency of the oscillation. In such cases, the amplitude can be changed by providing radially displaceable eccentric masses on a rotating shaft. So that the amplitude will not be changed during starting and stopping, or forward and reverse traversing, the position of the eccentric mass should be independent of the frequency of the shaft as well as its direction of rotation, otherwise depressions can easily occur when rolling asphalt paving, for example.
It is already known to dispose an eccentric mass displaceably and lockably in a cylindrical chamber. The displacement is provided by a hydraulic liquid acting on the eccentric mass formed as a piston sealed tightly against the cylinder wall, the position of the mass in the cylinder being determined by the quantity of liquid pumped in. The position of the eccentric mass is thus independent of both frequency and direction of rotation. Ground vibrators with eccentric elements of the kind mentioned will be complicated and expensive since a separate hydraulic system is required for operating the piston. Furthermore, the packings in the system are subjected to large stresses and therefore they often leak and give inexact positioning.
It is also known to arrange two radially displacable eccentric masses mutually lockable with the aid of a solenoid or locking pin. The eccentric masses are not commonly lockable however, but are kept in their rest position by a spring means which is dimensioned such that the centrifugal force overbalances at a certain rotational speed, one or both eccentric masses thus assuming a displaced position. This eccentric apparatus will also be relatively complicated, and the electrical or mechanical locking device will be subjected to large stresses due to shaking or vibration.
The present invention has the object of providing an eccentric element suitable for ground vibrators, and including a lockable eccentric mass, the displacement of which is independent of the direction of rotation and is not altered by alterations in speed of rotation, and which is also not burdened by the disadvantages accompanying the known eccentric elements mentioned.
The eccentric element in accordance with the invention includes an eccentric mass disposed radially movable on a rotatable shaft. Due to the rotation of the shaft, the mass can be caused to assume a position, displaced in relation to the centreline of the shaft. What is particularly characteristic for the invention is that the eccentric mass is lockable in its end positions by the action of a displaceable locking piston arranged in a recess in the rotating shaft and which can be controlled to assume a position thrust completely into the recess, wherein the eccentric mass is released, or a thrust-out position in which the locking piston fixes the eccentric mass in one of its end positions.